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IC 


9095 



Bureau of Mines Information Circular/1986 



Safe Cleaning of State of Maine Filters 
Using EDTA-Type Chelating Agents 

By John E. Pahlman and Daniel N. Tallman 




UNITED STATES DEPARTMENT OF THE INTERIOR 



c 



-^JjtaM^. B^g^jj^^f^2 



Information Circular 9095 



Safe Cleaning of State of Maine Filters 
Using EDTA-Type Chelating Agents 

By John E. Pahlman and Daniel N. Tallman 




UNITED STATES DEPARTMENT OF THE INTERIOR 
Donald Paul Hodel, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 







\\r 



v) M - «s 



4 



Library of Congress Cataloging in Publication Data: 



Pahlman, J. E. (John E.) 

Safe cleaning of State of Maine filters using EDTA-type chelating 
agents. 

(Information circular ; 9095) 

Bibliography: p. 12 . 

Supt. of Docs, no.: I 28.27:9095 

1. Filters and filtration. 2. Ethylene-diaminetetraacetic acid. 3. Ore dressing. 4. Calcium 
carbonate. I. Tallman, Daniel N. II. Title. III. Series: Information circular (United States. 
Bureau of Mines) ; 9095. 



TN295.U4 [TN535] 622 s [669'.22] 86-600226 



CONTENTS 



Abstract 1 

Introduction 2 

Acknowledgment 2 

State of Maine filter system 3 

Characterization of filter blinding 5 

Laboratory test procedures and results 7 

Filter cleaning strategies with chelating agents 9 

Field test procedures and results 9 

Summary and conclusions 12 

References 12 

ILLUSTRATIONS 

1. State of Maine, 300-st/d, filter system 3 

2. Generalized flow diagram for State of Maine filter system operation 4 

3. Schematic diagram of State of Maine filter unit 4 

4. SEM micrograph of blinded filter socks 6 

5. EDAX data for blinded filter sock 7 

6. Effect of Na4EDTA solution concentration and agitation on CaC03 removal.. 8 

7. Schematic diagram of Na4EDTA recirculation system 10 

8. Weight of CaC03 removed from blinded filters as a function of time 10 

9. Schematic diagram of in situ Na4EDTA system plumbing modifications 11 

TABLE 

1. Scale removal data from laboratory evaluation of chelating agents 7 





UNIT OF MEASURE ABBREVIATIONS USED IN 


THIS REPORT 


°c 


degree Celsius 


mg/L 


milligram per liter 


g 


gram 


min 


minute 


gal 


gallon 


mL 


milliliter 


gal/min 


gallon per minute 


pet 


percent 


g/L 


gram per liter 


psig 


pound per square inch, 
gauge 


h 


hour 










rpm 


revolution per minute 


in 


inch 










st/d 


short ton per day 


KeV 


kilo electron volt 










tr oz/st 


troy ounce per short 


L 


liter 




ton 



SAFE CLEANING OF STATE OF MAINE FILTERS 
USING EDTATYPE CHELATING AGENTS 



By John E. Pahlman 1 and Daniel N. Tallman 2 



ABSTRACT 

An increasing number of gold and silver mining operations are employ- 
ing cyanide solutions to effectively dissolve silver and gold from 
finely disseminated ores. Many of these operations use small, packaged 
processing systems like the State of Maine filter system, which employs 
zinc precipitation of gold and silver. Solution-clarifying filters in 
these systems become blinded with calcium carbonate (CaC0 3 ) scale and 
require frequent (every 2 to 10 days) dismantling of the filter units 
for acid cleaning. This is a hazardous operation as it is not compat- 
ible with the alkaline cyanide leach solutions and may result in the 
evolution of toxic hydrogen cyanide (HCN) gas during cleaning and during 
any accidental acid spill. 

The Bureau of Mines has investigated an alternative, nonacidic method 
of cleaning blinded State of Maine filters that employs ethylene- 
diamine-tetraacetic-acid (EDTA) type chelating agents to dissolve the 
scale. In situ cleaning of a 150-st/d State of Maine filters with EDTA- 
type chelating agents was successfully demonstrated. Greater than 
90 pet of the CaC0 3 scale was removed in about 10 min with circulation 
of 31 L of 2.5-pct-EDTA solution through the filter unit. The EDTA 
cleaning method not only eliminates the cyanide hazard in filter clean- 
ing but is advantageous from the standpoint of operating costs and 
productivity. 

Supervisory physical scientist. 

2 Research chemist (now with Economics Laboratory, Eagan, MN). 
Twin Cities Research Center, Bureau of Mines, Minneapolis, MN. 



INTRODUCTION 



Cyanide solutions are widely used in 
precious metal ore processing to effec- 
tively dissolve finely disseminated gold 
and silver. Chamberlain (JJ 3 identified 
over 80 operations that were actively 
leaching or seriously planning on cyanide 
leaching of gold or silver by either 
dump, heap, or in situ leaching. Contact 
of cyanide-containing solutions with free 
silver and gold results in the following 
reactions (2-3) : 



2Ag -+ 4NaCN + 2 + H 2 

+ 2NaAg(CN) 2 + H 2 2 + 2NaOH (A) 
4Ag + 8NaCN + 2 + 2H 2 

+ 4NaAg(CN) 2 + 4NaOH. (B) 



and 



These reactions strongly depend upon the 
amount of oxygen in solution. Thus, oxy- 
gen is added to the leach solution by 
bubbling air through it and/or by spray- 
ing it onto heaps. 

Recovery of gold and silver from preg- 
nant leach solutions can be accomplished 
by carbon adsorption with subsequent 
stripping and electrowinning of the gold 
and silver, or by the Merrill-Crowe re- 
covery process, which involves precipi- 
tation of gold and silver with zinc dust. 
Both methods have their respective ad- 
vantages and disadvantages. Selection of 
the recovery method depends upon the size 
and specific conditions of the leach- 
ing operations and facilities already 
available. One disadvantage of the zinc 



precipitation method, with respect to the 
carbon adsorption method, is the need to 
filter out suspended particles from the 
pregnant leach solution before zinc pre- 
cipitation. This is necessary to prevent 
the coating of the zinc particles by 
the suspended particles and consequent 
retardation of the precipitation reac- 
tion. Capital expenditures to establish 
a conventional mill circuit system based 
on the Merrill-Crowe process are high, 
but there are now on the market, small 
prepackaged mill circuit systems that are 
designed and priced for small leaching 
operations. One of these is the State of 
Maine (SOM) filter system. 

One problem encountered with opera- 
tional SOM filter systems is irreversible 
plugging of tbe filter every 2 to 10 days 
of operation. This necessitates disman- 
tling of the filter unit and cleaning 
of the filters with acid, a potentially 
hazardous operation because of the evolu- 
tion of toxic HCN gas. Mine personnel 
who breathe or come into skin contact 
with the toxic HCN gas can develop cyano- 
sis. HCN is a chemical asphyxiant that 
exerts its effect by inhibiting the meta- 
bolic enzyme systems (4^). Mild cyanosis 
results in nausea, while severe cyanosis 
can result in asphyxia and death. 

As part of its present research, the 
Bureau of Mines sought to determine what 
caused the blinding of the SOM filters, 
and to eliminate the cyanide hazard by 
devising a safe method of filter cleaning 
that does not evolve toxic HCN gas. 



ACKNOWLEDGMENT 



The authors wish to thank Charles Esca- 
pule, Louis Escapule, Bailey Escapule, 
and other personnel at Tombstone Silver 
Mines, Inc., for allowing the Bureau to 



fer to items in the list of references at 
the end of this report. 



field test the EDTA cleaning method on 
Tombstone's State of Maine filters, and 
for their assistance in setting up the 
necessary equipment to perform the tests. 
The authors also wish to thank Dow Chemi- 
cal Co. for supplying the Bureau with 
many of the chelating agents employed in 
this study. 



STATE OF MAINE FILTER SYSTEM 



Figure 1 shows a 300-st/d SOM filter 
system. Typically, a large SOM filter 
system consists of two filter units for 
clarification and two filter units for 
zinc precipitation. Depending upon the 
size of the leaching operation, smaller 
filter units containing 72 filter socks 
or larger filter units containing 120 
filter socks are employed. For continu- 
ous operation, a system will have two 
filter units in parallel for both the 
clarification and zinc precipitation 
operations so that one filter can be 
cleaned while the other is in operation. 
The SOM filter system also consists of a 
deaeration tank, a zinc dust feeder, and 
the necessary vacuum and liquid pumps, 
valves, and other plumbing to carry out 
the process. 

Figure 2 gives a generalized flow dia- 
gram for the SOM filter system operation. 



First, pregnant leach solution is pumped 
from the holding pond to the SOM filter 
system. Next, suspended particulates are 
removed by the first clarifying SOM fil- 
ter unit. The filtered solution then 
passes through the deaeration tank and 
zinc dust is added before the solution 
enters the second SOM filter unit, where 
silver (or gold) precipitation proceeds 
by the following reaction in the second 
SOM filter unit: 

2NaAg(CN) 2 + 2Zn ■* Na 2 Zn(CN) 4 + 2Ag (C) 

2NaAu(CN) 2 + 2Zn •> Na2Zn(CN) 4 + 2Au (D) 

Failure to remove oxygen from the leach 
solution results in the redissolving of 
precipitated silver and gold and precipi- 
tation of zinc oxide (ZnO) in the second 
SOM filter unit. 




FIGURE 1.— State of Maine, 300-st/d, filter system. 




To heap <- 



NaCN CaO 
-J I- 



Barren solution 
makeup tank 



Zn dust 
feeder 



Precipitation 




SOM 
filter 
unit 



Smelting 
furnace 



I Ag, Au precipitate 



FIGURE 2.— Generalized flow diagram for State of Maine filter system operation. 



The zinc replaces the silver (gold) in 
the cyanide complex. Enough free cyanide 
must be present in solution to form a 
soluble zinc complex to keep the zinc 
metal surface free to contact fresh solu- 
tion. Hydrogen gas is produced by zinc 
oxidation as a byproduct of the precip- 
itation of silver (gold). Finally, the 
silver-free (gold-free) barren solution 
is pumped to a makeup tank where cyanide 
level and pH are adjusted before again 
being sprayed on the leach pad. The pre- 
cipitate is dried and smelted to produce 
an ingot of silver (gold) . 

The SOM filter units are basically 
swimming pool filter canisters adapted 
for the SOM filter systems. (See figure 
1.) The 150-st/d SOM filter system em- 
ploys a smaller filter unit that contains 
72 nylon filter socks about 13 in long, 
while the 300-st/d SOM filter system em- 
ploys a larger filter unit containing 120 
nylon filter socks of similar size. Fig- 
ure 3 shows a schematic diagram of a SOM 
filter unit. Turbid pregnant leach solu- 
tion is pumped into the first SOM filter 
unit and forced through the filter socks. 



Bumping handle 




FIGURE 3.— Schematic diagram of SOM filter unit. 

Clay and other suspended particles are 
retained, and the clarified solution then 
passes to the deaeration tank. Zinc dust 
is added to the deaerated and clarified 
pregnant leach solution, which is then 
pumped into the second SOM filter where 
precipitated silver (gold) is recovered. 



The drain is employed to remove spent 
diatomaceous earth and precipitated metal 
values from the clarification and precip- 
itation filters, respectively. 

Diatomaceous earth is used as a precoat 
on the filter socks of the clarifying 
filter to facilitate filtering. When the 
diatomaceous earth is blinded due to en- 
trapped clay and suspended particles, the 
filter is manually "bumped" (slight re- 
verse flow generated through the filter) 
to redistribute the diatomaceous earth, 
and filtering is continued. When "bump- 
ing" no longer improves solution flow, 
the mixture of diatomaceous earth, clay, 
and suspended particles is "bumped" off 
the socks and removed from the filter via 
the drain. A new diatomaceous earth pre- 
coat is applied, and the procedure is 
started again. All this is done without 
opening the filter units. The filter 
units are opened for cleaning only 
when the filter socks are irreversibly 
blinded. Depending upon varying condi- 
tions such as water hardness and tempera- 
ture, this may be necessary in as little 
as 2 days. From the standpoint of health 
hazard (the less frequent the exposure, 
the better) and production (the less 
downtime, the better), it would be ideal 
if opening of the filter units were 



only required when a filter sock was rup- 
tured and needed replacement. Irrevers- 
ible blinding of the filter socks of the 
precipitation filter units also occurs, 
but much less frequently. 

The common procedure for cleaning the 
filter socks is to (1) drain the filter 
canister, (2) open the filter assembly, 
(3) raise the filter tubes and socks 
out of the unit to drain off any solu- 
tion, (4) rinse the socks with water, 
(5) rub the socks, (6) soak overnight 
in a dilute acid bath (HC1), and finally 
(7) rub and rinse the filter socks 
before replacing them in the filter unit 
again. An alternate procedure for clean- 
ing SOM filters consists of dipping them 
in an alkaline hypochlorite solution 
after following the first four steps of 
the above procedure, then dipping them in 
an acid bath, and finally dipping them in 
a lime solution before replacing them in 
the filter unit. Dipping the filters in 
the alkaline hypochlorite solution should 
oxidize any cyanide to innocuous CNO". 
Incomplete oxidation and hydrolysis in 
this step result in the evolution of 
HCN and cyanogen chloride (CNC1) in the 
acid bath. CNC1 is also toxic and can 
cause severe chemical burns and pulmonary 
edema. 



CHARACTERIZATION OF FILTER BLINDING 



Blinded filter socks were cut into 3/4- 
by 3/4-in (1/4-g) coupons for analysis 
to determine the nature of the substance 
plugging the filters. Several of these 
coupons were prepared for examination 
under the scanning electron microscope 
(SEM). Other coupons were reacted with 
100 mL of 1:1 HC1 solution overnight. 

Examination of the blinded filter sock 
under the SEM showed the presence of many 
crystallites of an unknown compound (fig. 
4) . Elemental analysis of this blinded 
filter sock using energy dispersive an- 
alysis of X-rays (EDAX) indicated that 
these crystallites were mainly composed 
of calcium with traces of zinc, iron, 
and silver (fig. 5). The presence of 
silicon, aluminum, and magnesium is due 
to the clay and/or diatomaceous earth 



trapped on the filter. Reacting four 
scaled filter coupons overnight with 
100 mL of 1:1 HC1 solution resulted in a 
solution containing about 3.6 g/L Ca, 
about 30 mg/L Zn, and about 20 mg/L Fe. 
Evolution of gas bubbles during acid 
attack of the filter coupons indicated 
that the plugging substance was a carbon- 
ate compound. From the acid-digestion 
analysis data, the SEM examination, and 
the EDAX analysis data, it was concluded 
that the substance that irreversibly 
plugs the filter is mainly CaC0 3 scale. 

From the standpoint of clay and 
suspended particles removal, the SOM fil- 
ter unit appears to be more than adequate 
when precoated with diatomaceous earth. 
Thus, alternative filter types that 
can be cleaned by backwashing were not 




FIGURE 4.— SEM micrograph of blinded filter socks (X600). 



_ 






1 1 




I 1 


_ 


- 




Co 

1 






- 


- 












- 


- 


Al 
M9 .d 


Ag [ 
ill 


Co 

A 






- 




/ 


Uy/^ 


Vi/Wntv, 


Fe 


-vkW^Wj 


v^H\ 



X-RAY ENERGY, KeV 

FIGURE 5.— EDAX data for blinded filter sock. 



evaluated, as these types of fil- 
ters would not solve the problem of 
calcium-scaled filters. Calcium scaling 
would also blind other filters and would 
probably not be removed by backwashing. 

Based on discussions with mining com- 
pany personnel who were using SOM filter 
units and on the nature of the plugging 
scale, it was concluded that cleaning of 
these filters with nonacidic solutions 
(1) should be possible and (2) would 
drastically reduce the cyanide hazard. 
Cleaning of the filters in situ — i.e., 
without opening up the filter units — 
would be advantageous not only by elimi- 
nating the health hazard to mining com- 
pany personnel but also by increasing 
productivity, 



LABORATORY TEST PROCEDURES AND RESULTS 



Although there are many scale inhibi- 
tors and removers on the market, the 
present research was restricted to an 
evaluation of the common EDTA-type che- 
lating agents as to their effectiveness 
in removing scale from blinded filters. 
The seven chelating agents tested were 

(1) NasDTPA, a pentasodium salt 
of diethylene-triamine-pentaacetic acid; 

(2) Na4EDTA, a tetrasodium salt 
of ethylene-diamine-tetraacetic acid; 

(3) Na3HEDTA a trisodium salt 
of n-hydroxyethyl-ethylene-diamine-tri- 
acetic acid; 

(4) (NH.4)2EDTA diammonium, disodium 
salt of ethylene-diamine-tetraacetic 
acid; 

(5) (NH.4)4EDTA, tetraammonium salt 
of ethylene-diamine-tetraacetic acid; 

(6) (EGTA) ethylene-glycol-bis(amino- 
ethyl)-tetraacetic acid; and 

(7) (TEA) tri-ethanolamine. 

Initial evaluation tests involved 
overnight shaking of 100 mL of 5-pct so- 
lution of each chelating agent with 
four scaled filter coupons (total weight, 
about 1 g). After this chelation treat- 
ment, the residual filter coupons were 
dried and then reacted with 100 mL of 1:1 
HC1 to remove any residual scale, and 
thus to enable the determination of the 
extraction efficiency of each chelating 



agent. By employing a large excess of 
reagent for a relatively long time, a 
"pseudo-equilibrium" is set up and the 
selectivity of each reagent established. 
Results of these tests are shown in table 
1, where the respective pH values of each 
5-pct chelating agent solution and the 
percent removal of calcium, zinc, and 
iron are given. 

Except for TEA, which removed neglig- 
ible calcium, the other six chelating 
agents removed from 96.9 to 99.3 pet of 
the calcium present, thereby indicating 
that all these agents could be used 

TABLE 1. - Scale removal data from 
laboratory evaluation of chelating 
agents 



Reagent 



Na 5 DTPA 

Na 4 EDTA 

Na 3HEDTA 

(NH 4 ) 2EDTA. . 

(NH 4 )4EDTA. . 



EGTA. 
TEA.. 



pH 



12.5 

12.5 

12.5 

5.2 

'12. 1 

8.9 

'12.1 

12.4 

10.0 



Removal, pet 



Ca 



98.8 
99.3 
98.3 
99.1 
98.6 
99.2 
98.7 
96.9 
.3 



Zn 



63.5 
64.6 
63.5 
84.0 
75.8 
72.1 
75.0 
12.9 
2.7 



Fe 



6.4 

3.6 

13.6 

21.8 

2.2 

13.0 

2.2 

1.6 

5.4 



'Adjusted pH. 



to remove calcium scale from the SOM 
filters in reasonable time. The best 
overall chelating agents for calcium, 
zinc, and iron removal are (NH4)2EDTA and 
(NH 4 ) 4 EDTA; however, the low pH of their 
solutions is not compatible with the nor- 
mal cyanide leaching solutions at pH 12. 
(NH 4 ) 2 EDTA and (NH 4 ) 4 EDTA do not perform 
as well at pH 12.1 (table 1), although 
the difference is slight. They are not, 
however, enough better at pH 12.1 than 
the other four good chelating agents to 
justify their extra cost and the trouble 
and cost of raising their pH to 12.1. 

Na 4 EDTA was chosen for further labora- 
tory testing and field evaluation studies 
based on the ease of solution prepara- 
tion, pH of chelating agent solution, 
calcium removal efficiency, zinc removal 
efficiency, and reagent cost. 

Tests were conducted using Na 4 EDTA to 
determine both the concentration of solu- 
tion required to remove the scale in a 
reasonable amount of time and whether so- 
lution agitation was advantageous. For 
these tests, two 250-mL aliquots each 
of 2.5-pct, 5.0-pct, and 10.0-pct Na 4 EDTA 
were reacted with eight scaled filter 
coupons (2 g) . For analysis, 25 mL of 
solution was withdrawn after 5, 10, 20, 
40, and 60 min of reaction. One set of 
2.5-pct, 5.0-pct, and 10-pct Na 4 EDTA so- 
lutions was stirred at a rate of 100 rpm, 
while the other set was not stirred. The 
reacted coupons were dried and then 
immersed in 100 mL of 1:1 HC1 solution to 
determine calcium removal efficiency at 
each time interval for each solution 
concentration. 

Figure 6 graphically shows the results 
of the tests. For nonstirred solutions, 
95-pct calcium removal was achieved in 
60, 45, and 40 min, respectively, for the 
2.5-pct, 5.0-pct, and 10.0-pct-Na 4 EDTA 
solutions. For stirred solutions, 95-pct 
calcium removal was achieved in less than 
10 min for all three solution concentra- 
tions. On the basis of these tests, it 
was concluded that the filter unit could 
be cleaned in about 10 min with any of 
the three solution concentrations if agi- 
tation was provided. 

Further evidence of the ability of 
Na 4 EDTA solutions to remove scale from 
the SOM filters was obtained by coating 




FIGURE 6.— Effect of Na 4 EDTA solution concentration and 
agitation on CaC0 3 removal. 

the outside of a 3-in-long specimen of 
filter sock with finely ground scale 
until a vacuum reading of 2.42 psig was 
obtained when attempting to draw water 
through the filter sock. A total of 5 g 
scale, which was obtained from other 
areas of the leaching and recovery cir- 
cuits, was coated on the filter sock. 
This scale's composition was similar to 
that present on plugged filters. Next, 
100 mL of a 10-pct-Na 4 EDTA solution was 
heated to 80° C and then introduced into 
the test compartment on the outside of 
the plugged filter sock. Initially, the 
solution was very slowly drawn through 
the filter sock; however, within 2 min 
the vacuum reading was greater than 13.7 
psig and the scale was almost totally 
removed. 



FILTER CLEANING STRATEGIES WITH CHELATING AGENTS 



Four main strategies are proposed for 
cleaning State of Maine filters using 
chelating agents: 

1. The chelating agent solution is 
simply substituted for the HC1 solution 
presently used to clean the filters. The 
disadvantage of this method with respect 
to the following strategies is that the 
operators are still exposed to cyanide 
solutions as they dismantle the filter 
units, remove clogged filters, and then 
rinse and rub them before dipping them 
into the chelating agent solution. 

2. The mixture of diatomaceous earth, 
clay, and suspended particles is drained 
from the filter unit. The unit is then 
disconnected from the filter system and 
reconnected to a separate recirculating 
pump system with hoses leading to and 
from a reservoir of chelating agent solu- 
tion. The filters are cleaned inside the 
filter unit by circulating the chelating 
agent solution throughout the unit. This 
treatment method is safer than method 1, 
since the operator does not have to re- 
move and handle the filter. A second 
advantage is that the extra wear on the 
filter units associated with torquing 
down the bolts is avoided since the fil- 
ters are not removed from the filter 
unit. 



3. Blinded filters are cleaned in situ 
by circulating chelating agent solution 
through the filter unit after drain- 
ing the leach solution. This method has 
the same advantages of method 2 with 
respect to method 1. The advantages of 
this treatment method over method 2 are 
that the filter unit does not have to 
be disconnected, moved, and reconnected. 
The plumbing modifications needed to add 
a recirculating loop for the chelating 
agent solution are minimal. 

4. In this method, scale formation is 
controlled or avoided by continuously 
metering small doses of chelating agent 
to the pregnant leachate as it enters the 
filter unit. The effect of this treat- 
ment on the Merrill-Crowe process over 
extended periods of time is not known. 
Elevated levels of soluble EDTA-calcium 
complexes can be expected in the leaching 
system. An advantage of this method over 
the other three methods is that there 
would be no downtime required for filter 
cleaning. A second advantage of this 
treatment method over treatment methods 2 
and 3 is that a separate recirculation 
system for filter cleaning is not re- 
quired. Plumbing modifications of the 
system for slowly dosing the chelating 
agent are minor. 



FIELD TEST PROCEDURES AND RESULTS 



Five tests were conducted to validate 
laboratory results on actual filter 
units. Two tests each were conducted to 
evaluate treatment method 1 and treatment 
method 2, respectively, while one test 
was conducted to evaluate treatment 
method 3. 



these filters was low, these tests indi- 
cated that the chelating agent solution 
could be used, rather than the HC1 
solution, for filter cleaning. Unfortu- 
nately, more filters with heavy scaling 
were not available at the time of 
testing. 



Method 1 (Test 1 and 2) 



Method 2 (Tests 3 and 4) 



Two filters from a 300-st/d unit (120 
filter socks) were cleaned by dunking 
them into a 30-gal reservoir of 2.5- 
pct-Na 4 EDTA solution for 15 min. The 
filters appeared to be relatively clean 
before the tests, though 7.1 g and 2.4 g 
CaC0 3 were removed from them in less than 
15 min. Although the amount of scale on 



Two filters from a 150-st/d unit 
(72 filter socks) were cleaned using 
treatment method 2. Figure 7 shows the 
Na4EDTA recirculating system. It con- 
sists of a solution reservoir (25 gal) , 
an intake hose, a discharge hose, and a 
recirculating pump to pump the cleaning 
solution through the filter unit. With 



10 




FIGURE 7.— Schematic diagram of Na 4 EDTA recirculation 
system. 



a scaled filter in place (test 3), the 
filter unit was connected to the dis- 
charge hose and the recirculating pump. 
The intake hose was connected to the 
pump. Water was first circulated through 
the filter to determine the pressure drop 
across the blinded filter. A steady 
pressure drop of 19 psig was observed. 
The water was drained from the filter 
unit, and pumping of Na4EDTA solution 
through the filter unit was begun. Dur- 
ing the first 10 min of the test, the 
solution coming into the reservoir 
through the discharge hose was turbid. 
(It was clear for water circulation.) 
During this time, the pressure drop 
across the filter decreased fairly 
quickly from 19 psig to 3-5 psig, and re- 
mained in that range until the completion 
of the test. 

Figure 8 shows the amount of CaC03 dis- 
solved as a function of time for tests 
3-5. In test 3, CaC0 3 weight removal was 
calculated from the calcium analysis of 
the Na4EDTA reservoir solution samples 
taken 1.5, 2.5, 4.5, 10, 20, and 30 min 
after the Na4EDTA solution was first in- 
troduced into the filter unit (figure 8, 
curve A). As shown, the cleaning was 
about 90 pet complete in 10 min, and a 
total of 87.4 g CaC03 was removed from 
the filter. For all practical purposes, 
the filter is probably unblinded in the 
time it takes to fill the filter, about 1 
to 2 min. It appears that some scale 
particles are loosened from the filter 
but do not immediately dissolve. This 
gives rise to the initial turbidity of 
the Na4EDTA solution in the reservoir; as 
a result, initial values calculated for 
calcite removal based on analysis of 







1 




1 




80 






A 


A 


- 


70 






B^~- — 




" 


60 


"A J 








- 


50 








KEY 
° Test 3 (A) 
A Test 4 IB) 


- 


40 








• Test 5 IC) 


- 


30 










- 


20 
10 






C 


• 




I 


1 


1 


I I 


- 



FIGURE 8.— Weight of CaC0 3 removed from blinded filters 
as a function of time. 



dissolved calcium are lower than the 
amount of scale actually removed from the 
filter. 

Test 4 was similar to test 3, except 
that the filter used was not as scaled; 
the same cleaning solution was used as in 
test 3. But in this test only 20 g CaC0 3 
was removed from the filter, with clean- 
ing essentially complete in less than 
5 min (figure 8, curve C) . This differ- 
ence can be attributed to the fact that 
clean water pressure drop across this 
partially blinded filter was only 2 to 3 
psig over that for the filter after it 
had been cleaned with Na4EDTA. 

Method 3 (Test 5) 

The required Na4EDTA recirculating sys- 
tem was connected directly to the exist- 
ing plumbing of an operating 150-st/d 
filter unit. Figure 9 shows a schematic 
of the Na4EDTA system plumbing with 
respect to that of the operating unit. 
The filter in the unit was not totally 
blinded, as a pressure drop of only 
10 psig was observed for an operating 
flow rate of 35 to 40 gal/min. To see a 



11 




I From No, EDTA 




FIGURE 9.— Schematic diagram of in situ Na 4 EDTA system 
plumbing modifications. 



greater change in calcium concentra- 
tion throughout the test, the volume of 
Na4EDTA being recirculated was decreased 
to about 31 L (8.2 gal). A 5/8-in-diam 
garden hose was used instead of the 1- 
and 2-in-diam hoses used in testing of 
method 2. Samples were taken from the 
solution reservoir 1.5, 2.5, 4, 6, 8, 10, 
12, 15, and 20 min after introduction of 
the cleaning solution into the filter 
unit. In contrast to previous tests, the 
filter socks were not rinsed to remove 
adherent diatomaceous earth after the 
leach solution was drained from the fil- 
ter unit. A fair amount of diatomaceous 
earth precoat probably remained on the 
filter. This same procedure would be 
used in future cleaning operations. 

In test 5, using method 3, cleaning was 
slower than for tests 3 and 4 using meth- 
od 2; CaC03 was still being removed, 
although slowly, at the end of the 20-min 
test period (figure 8, curve B) . This ob- 
served decrease in cleaning efficiency 
may be attributed to the slower recircu- 
lation rate employed in these tests; 
however, it is more likely attributable 
to the presence of the diatomaceous earth 
remaining on the filter socks. The po- 
rous structure of the diatomaceous earth 
is what makes it useful as a precoat, and 
the cleaning rate for removing calcite 
from diatomaceous earth is probably dif- 
fusion limited. SEM examination of dia- 
tomaceous earth from a scaled filter 
showed the presence of calcite in its 
pores. Thus, the observed increase in 
calcite removal from 15 to 20 min into 
the test is believed to be attributable 
to calcite removal from the diatomaceous 
earth remaining in the filter unit. 



Cleaning the diatomaceous earth of 
CaC03 does not appear to be important, 
since the diatomaceous earth is discarded 
anyway. Assuming that much of the cal- 
cium dissolved after 15 min of testing is 
attributable to removal of calcite from 
the diatomaceous earth, >90 pet of the 
calcite on the filter is removed in 10 
min. As was the case in the tests of 
method 2, the reservoir was turbid in the 
first few minutes of the operation. In 
fact, calcite particles could actually be 
seen in the 1.5-min sample of Na4EDTA so- 
lution as it was being collected. By the 
time the samples were filtered for analy- 
sis, the turbidity of the 1.5-min sample 
had disappeared. The high calcite re- 
moval value for the 1.5-min sample indi- 
cates that the filter is probably in 
effect unblinded within several minutes, 
even though not all the calcite that is 
removed from the filter is dissolved. 
Total calcite removal from this filter 
was 73.4 g. 

Method 4 

Method 4 was not field tested as part 
of this investigation; however, bench- 
scale leaching tests were conducted 
at the mine to determine the effect, 
if any, of Na4EDTA on cyanide leaching. 
For these tests, 40 g of minus 200-mesh 
silver ore, 75 mL of cyanide leaching so- 
lution (0.05 pet CN~), and either 12.5 mL 
of distilled water or 12.5 mL of 2.5-pct 
Na4EDTA were shaken in stoppered flasks 
for 1 h. The amount of Na4EDTA added 
was that amount needed to complex all 
the calcium added in the leach solution 
for pH control. Silver extractions of 
1.5 tr oz/st were obtained in both leach 
tests, thereby indicating that the pres- 
ence of chelating agent in the leaching 
circuit should not adversely affect the 
cyanide leaching process. Any additional 
problems in the leaching and recovery 
processes attributable to dosing small 
amounts of Na4EDTA solution into filter 
units will become apparent only after a 
long-term evaluation of method 4. 



12 



SUMMARY AND CONCLUSIONS 



In situ cleaning of SOM filters with 
ethylene-diamine-tetraacetic acid (EDTA) 
chelating agent has been successfully 
demonstrated. Greater than 90 pet of the 
calcium-base scale was removed in about 
10 min with circulation of about 31 L of 
2.5-pct-Na 4 EDTA solution through the 150- 
st/d-size filter unit. This nonacidic 
method replaces the old acid (HC1) clean- 
ing method, in which the filter unit had 
to be dismantled and the filter socks re- 
moved, rinsed, rubbed, soaked in dilute 
acid, and then rubbed and rinsed again 
before being reinstalled in the filter 
unit. This eliminated the health and 
safety problem associated with acid 
cleaning of the filters; i.e., toxic HCN 
evolution during cleaning and during an 
accidental acid spill. 

In addition to the elimination of the 
cyanide hazard in filter cleaning, 
in situ cleaning of the filters with EDTA 
solutions is advantageous from the stand- 
point of operating costs and productiv- 
ity. By not having to open the filter 
units to clean the filters, the wear and 
tear on the filter socks (due to rubbing 



and acid degradation) is eliminated, and 
the wear and tear on the filter unit 
(degradation of rubber gaskets and crack- 
ing of the filter unit tops due to over- 
torquing bolts) is drastically reduced. 
Furthermore, the downtime for a filter is 
reduced from 30 to 40 min to 10 to 15 min 
by the EDTA cleaning method. 

Since lime is often added to maintain 
high pH in cyanide leaching systems, 
calcite (CaC03) scaling of filters and 
plugging of sprayer nozzles and plant 
piping are common problems for many cya- 
nidation operations. The carbon-in-pulp 
(CIP) system for recovery of silver and 
gold from pregnant cyanide leach solu- 
tions also has scaling problems. The 
carbon particles are rendered ineffective 
due to calcite scaling and must be 
periodically cleaned. Use of chelating 
agents should be generally applicable 
to cleaning of other filter types, un- 
plugging sprayer nozzles and plant pip- 
ing, and cleaning carbon particles as 
well as to the cleaning of State of Maine 
filters. 



REFERENCES 



1. Chamberlain, P. G. , and M. G. Po- 
jar. Gold and Silver Leaching Practices 
in the United States. BuMines IC 8969, 
1984, 47 pp. 

2. Barsky, G. , S. J. Swainson, and 
N. Hedley. Dissolution of Gold and 
Silver in Cyanide Solutions. Trans. 
AIME, v. 112, 1934, pp. 660-677. 

3. Duncan, D. M. Open Pit Gold Min- 
ing at Cortez. Ch. in Colorado Mining 



Association 1974 Mining Yearbook. CO 
Min. Assoc, 1974, pp. 92-94. 

4. Mackinson, F. W. , R. S. Stricoff, 
and L. J. Partridge (eds.). Occupa- 
tional Health Guidelines for Chemical 
Hazards. Arthur D. Little, Inc., 
DHHS(NIOSH) 81-123, 1981, 5 pp. 



4 U.S. GOVERNMENT PRINTING OFFICE: 1986—605-017/40,069 



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Bureau of Mines— Prod, and Distr. 
Cochrans Mill Road 
P.O. Box 18070 
Pittsburgh. Pa. 15236 



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